Title: The Hunt for Mobile Holes Induced by Polarization in GaN
Speaker: Prof. Huili Grace Xing (Cornell University, Ithaca, 14853, USA)
Time: Dec. 17, 2019  9:30 AM 
Venue: No. 320 meeting room of building 3, IOS, CAS  
Abstract: Two-dimensional electron gas (2DEG) in the GaN family can be readily induced by polarization discontinuity without impurity dopants at a heterointerface, which is the heart of a high-electron mobility transistor (HEMT). As a fundamental departure from impurity-doping and modulation-doping, polarization-induced three-dimensional electron gas in GaN was also postulated and experimentally demonstrated for the first time in 2002 [1]. Thanks to the advent of high-quality bulk GaN substrates, polarization-induced three-dimensional hole gas in GaN was demonstrated in 2010, with assistance of impurity dopants; for the first time, p-type conductivity in GaN was measured down to cryogenic temperatures, 4 Kelvin [2]. It has been about 20 years since the existence of mobile holes in GaN heterostructures without impurity dopants, the counterpart of 2DEG, was postulated. Only recently, undisputable experimental observations are achieved in our lab [3]. The long-missing polarization-induced two-dimensional hole gas (2DHG) is finally observed in undoped gallium nitride quantum wells. Experimental results provide unambiguous proof that a 2D hole gas in GaN grown on AlN does not need acceptor doping, and can be formed entirely by the difference in the internal polarization fields across the semiconductor heterojunction. The measured 2D hole gas densities, about 4x1013 cm-2, are among the highest among all known semiconductors and remain unchanged down to cryogenic temperatures. The observed results provide a new probe for studying the valence band structure and transport properties of wide-bandgap nitride interfaces, and enable the missing component for gallium nitride-based p-channel transistors for energy-efficient electronics.
Biography:Huili Grace Xing is currently the William L. Quackenbush Professor of Electrical and Computer Engineering, Materials Science and Engineering at Cornell University. She was with the University of Notre Dame from 2004 to 2014. She received B.S. in physics from Peking University (1996), M.S. in Material Science from Lehigh University (1998) and Ph.D. in Electrical Engineering from University of California, Santa Barbara (2003), respectively. Her research focuses on development of III-V nitrides, 2-D crystals, oxide semiconductors, recently multiferroics & magnetic materials: growth, electronic and optoelectronic devices, including RF/THz devices, tunnel field effect transistors, power electronics, DUV emitters and memories. She has authored/co-authored 220+ journal papers and 100+ conference proceeding publications including Nature journals, Physical Review Letters, Applied Physics Letters, Electron Device Letter and IEDM etc. She is a recipient of AFOSR Young Investigator Award, NSF CAREER Award and ISCS Young Scientist Award. She is a fellow of APS.
1) D. Jena, H. Xing et al., Appl. Phys. Lett. 81(23), 4395-4397 (2002). DOI: 10.1063/1.1526161 
2) J. Simon, H. Xing, D. Jena et al., Science 327, 60 (2010) DOI: 10.1126/science.1183226 
3) R. Chaudhuri, H. Xing, D. Jena et al., Science 365, 1454 (2019) DOI:10.1126/science.aau8623